The present invention relates generally to radiation reflectors, and, more particularly, to frequency discrimination between multiple modulated reflectors for simultaneous remote position sensing, such as for in-flight refueling of aircraft.
Retroreflectors are well-known devices in the field of electromagnetic and optical radiation. A retroreflector, such as a corner reflector or corner cube, made by arranging three reflecting surfaces at mutual 90 degree angles, is often used as a cooperative target for locating and ranging because its signal return decreases as 1/R2, where R is range, instead of 1/R4, the performance of a normal flat mirror. Retroreflectors receive radiation energy and reflect such energy back in the direction from which the radiation energy came. Without modification, a retroreflector will return the radiation energy with the same general characteristics of amplitude versus time behavior as the incident radiation. Methods of modulating the characteristics of the amplitude versustime behavior of reflective radiation signals have been developed using modulated retroreflectors in different applications.
Presently, any modulation of radiation signals reflected from a retroreflector is caused by changing some characteristic of the retroreflector to alter its performance. For example, U.S. Pat. No. 6,233,088 to Roberson et al. discloses modulating a radiation signal by deforming a reflecting surface of the retroreflector.
By example only, one application of modulated retroreflectors is in the field of in-flight aircraft refueling. Pilots in manned aircraft use visual cues to locate, approach, and station a receiving aircraft relative to a tanker aircraft, at which point personnel on the tanker manipulate a boom to mate with the proper receptacle on the refueling aircraft or the pilot of the receiving aircraft approaches and mates to a drogue trailing from the tanker. This type of in-flight refueling operation requires skilled pilots in both aircraft to position and orient the aircraft and either a skilled boom operator to control the position of the refueling boom relative to the receiving aircraft or a skilled pilot to control the position of the receiving aircraft relative to the drogue. Because refueling operations are manually controlled at least in part, the chance of pilot or operator error increases the potential for a collision between the two aircraft or between the receiving aircraft and the refueling boom or drogue. Automation of these processes for unmanned aircraft requires location of cueing features on the tanker or drogue so an unmanned aircraft can perform the same process of locating a tanker and positioning itself relative to the tanker so that a skilled operator on the tanker may guide the boom into the desired position or relative to the drogue.
Other prior art systems for in-flight refueling of aircraft use optical systems with multiple reflectors. Each reflector may be distinguished from another by the pattern of their placements. Another method to distinguish the position of each of several retroreflectors is modulation of each reflected radiation signal at a different frequency. The different frequencies of modulation of the reflected radiation signals allow a frequency selective position measuring system to separately interpret the information provided by each reflected radiation signal in order to obtain precise measurement of the position of each retroreflector and, thus, the orientation of the structure upon which the retroreflectors are mounted. Presently, systems for simultaneously measuring positions of several reference points use an imaging system to observe positions of an array of retroreflectors distinguished by placement in a characteristic pattern. An alternative prior art method is the use of a non-imaging position measuring sensor system such as a duo-lateral photodiode position sensor that separately measures the different frequencies of modulation of radiation signals using electrical modulation of sources such as a light emitting diodes. This method can be extended to an array of retroreflectors by using electrical modulation of corner reflectors with a deformable reflecting surface, each at a characteristic frequency.
Typically two methods are used for implementing cooperative remote sensing: modulated or continuous light sources in a known array on the target, or modulated or unmodulated retroreflectors in a known array on the target. Unmodulated sources emit continuous wave signals and reflectors reflect incident signals back to an imaging position-sensing radiation detector and position determining system, which could be a pilot""s eyes or a video camera and image processor. Image sensors, such as video cameras, respond only to the intensity (and perhaps color) of light, as their frame rate is too slow to respond to modulation frequencies above a few hertz. Modulated sources and reflectors emit or reflect modulated signals back to non-imaging sensors. Non-imaging position sensors such as duo-lateral photodiodes have much faster speed of response, so they can measure the position of multiple sources if they are modulated at different frequencies so that the measurements can be separated based on frequency.
One prior art duo-lateral photodiode position sensor system uses light emitting diodes (LEDs), each emitting light modulated at different frequencies to enable a remote sensing device to measure the exact coordinate of each LED. However, using LEDs to create a coordinated source array requires electrical power and wiring to energize and modulate the emitted light of the LEDs. Another method of modulating a radiation signal is disclosed in U.S. Pat. No. 6,233,088 to Roberson et al. The Roberson method consists of selectively mechanically deforming at least a portion of a reflective surface of a reflector. As when using LEDs to produce a modulated radiation signal, the Roberson method, too, requires electrical power to produce a modulated reflected radiation signal. The necessity of a power source and the associated wiring to provide modulation for a remote sensing system can be a substantial disadvantage to implementing an automated remote sensing system for aerial refueling.
In order to reduce the risk of human life, unmanned air vehicles (UAV) have been employed and are being further developed for surveillance and smart weapon technology. An extension of an unmanned air vehicle is the use of such an aircraft for offensive and defensive combat strikes. The unmanned combat air vehicle (UCAV) was designed and is being further developed as a safe and affordable weapon system to expand the capabilities of an air defense and strike system. The goal of a successful UCAV design would provide the strength, features, and functionalities of a current strike fighter with limited human intervention by automating tasks that would have previously required human control. Ideally, a UCAV could perform all the tasks if not more tasks of a manned combat air vehicle, with limited or no human intervention required. For example, in order to be able to operate a UCAV during prolonged periods of time without landing, the vehicle would need to be refueled from a tanker aircraft. Thus it would be advantageous to design an automatic positioning system that is reliable enough to enable a UCAV to automatically position itself in relation to a tanker aircraft for refueling.
A fluid-motion-powered modulated retroreflector is therefore provided to modulate radiation signals in a unique manner to permit each retroreflector in an array to be discriminated and individually located. Such fluid-motion-powered modulated retroreflectors include a chopping wheel or a pair of polarizing filters, or polarizers, that may modulate the reflected radiation and a propeller to rotate a chopping wheel or one of the polarizing filters. Motion of the propeller through a fluid such as air causes rotational velocity of the propeller and eliminates the need for an independent drive mechanism to power the modulation of a reflected radiation signal. The apparatus includes a frequency selector to control the rotational velocity of the propeller, such as by limiting the maximum rotational velocity of the propeller. Fluid-motion-powered modulation is an alternative to using an electrical drive mechanism to rotate a chopping wheel or a polarizing filter, modulating reflected radiation signals, or otherwise changing the properties of an illuminated retroreflector.
Previous methods of modulating a retroreflector typically require electrical power to energize the means of modulation. For some applications it is desirable that the means of modulation not require electrical power or control. Furthermore, an array of modulated retroreflectors, each having their own predetermined modulation frequency, is desirable for difficult remote sensing applications. With such an array of modulated retroreflectors, each retroreflector reflects incident continuous wave signals at a different predetermined frequency and a position sensor, such as a duo-lateral photodiode, can measure the position of each modulated retroreflector simultaneously.
One embodiment of the present invention uses a chopping wheel to modulate the reflected radiation signal. A second embodiment of the present invention employs a fixed polarizing filter in combination with a rotating polarizing filter. In a further embodiment of the present invention, each reflector in an array of reflectors can reflect radiation that is differently or uniquely modulated to allow a position-sensitive radiation detector, or position-sensing photodetector, and position determining system or device to locate and determine the position of each reflector.
In accordance with one advantageous embodiment of the present invention, a chopping wheel with attached propeller blades is mounted in front of a reflector. When the propeller blades pass through a fluid, the chopping wheel rotates in front of the reflector. As incident radiation signals are reflect from the reflector, the chopping wheel blocks a varying portion of the incident and reflected signals to impose an amplitude modulation onto the reflected radiation signal. A frequency selector controls the modulation of the radiation signal by limiting the maximum rate of rotational velocity of the propeller blades. The frequency selector may be a governor similar to a combustion engine speed control device, a clutch or slip style device, or any other system designed to control the speed of rotation by fixing an upper limit. This embodiment of the present invention might also include a shaft, passing through or around the center of the reflector or reflector assembly, to which the chopping wheel and frequency selector are mounted. The chopping wheel may include any number of apertures to modulate the radiation signals.
Generally, the present invention is effective in air for use with aircraft. Alternatively, the present invention can be employed in other fluids including water for use with submarines and boats. The fluid and relative motion of the propeller is generally such that the propeller rotates with sufficient rotational velocity to be limited by the frequency selector.
In accordance with further principles of the present invention, a receiving aircraft emits a single incident radiation source towards a tanker aircraft mounted with modulated retroreflectors at different positions. This particular application of the present invention offers airflow to power the modulation of the retroreflectors. The incident radiation source is reflected by each of the reflectors and differently modulated in each instance. A position-sensitive radiation detector on the receiving aircraft receives each of the differently modulated reflected signals. A position determining device or system processes the signals from the position-sensitive radiation detector and determines the relative position of each of the retroreflectors based on the signal frequency components of the differently modulated reflected signals from each electrode of the position-sensitive radiation detector. This information can be provided to a control mechanism to automatically pilot the receiving aircraft into a desired, relative position with the tanker aircraft. The modulated retroreflectors might be positioned in a predetermined array to provide position and orientation determination and enable an automated system. A particular benefit of the present invention is the ability to differently modulate a single incident radiation signal with an array of differently modulated retroreflector signals.
In accordance with another advantageous embodiment of the invention, a rotatable polarizing filter is located in front of the reflector and a fixed polarizing filter is mounted before or behind the rotatable polarizing filter and in front of the reflector. Propeller blades extend outwardly from the rotatable polarizing filter. When the propeller blades pass through a fluid, or a fluid passes over the propeller blades, the propeller blades and polarizing material rotate in front of or behind the fixed polarizing filter and in front of the reflector. As radiation signals are received and pass through the rotatable and fixed polarizing filters, the radiation signals are linearly polarized and attenuated or possibly blocked if the polarizing axes of the two polarizing filters are crossed. After being reflected, the polarized radiation signal passes back through the polarizing filters. The constant rotation of the rotatable polarizing filter prevents portions of the signal from returning to the source due to different polarization angles of the fixed and rotatable polarizing filters. This signal blockage produces a similar modulated radiation signal, as does the blocking by the chopping wheel in the previously described embodiment. Again, a frequency selector can control the modulation of the a amplitude of the radiation signal by limiting the maximum rate of rotational velocity of the rotatable polarizing filter. Although not required, the frequency selector can also control the rotational velocity of the propeller blades that generate the rotational velocity of the rotatable polarizing filter.
As will be recognized by persons of ordinary skill in the art, numerous variations of propellers can be employed to produce rotation by fluid flow in a modulated reflector. Any type of configuration or shape of device that intercepts fluid flow to provide rotation would be appropriate to use with a reflector to create a fluid-motion-powered modulated retroreflector, including a squirrel cage with blades or a helical screw with no blades at all. Also for example, several fins on a cylindrical or conical surface may provide similar rotation, as can propeller blades. All such devices are generally referred to in this application as propellers or propeller blades, even such a device as a helical screw which, although absent of conventional physical propeller blades, functions as a propeller or propeller blade for purposes of this invention.
Also, as will be recognized by persons of ordinary skill in the art, numerous variations of chopping wheels can be employed to modulate the radiation signal. Any type of configuration of discs or size and shape of apertures in the discs that provide signal modification of the associated electromagnetic radiation would be appropriate to use with the present invention. And either the first or second chopping wheel disc need to rotate. All such devices are generally referred to in this application as chopping wheels. Furthermore, a patterned reflective coating such as a glass beaded painted surface, an array of small corner reflectors, a reflective tape, or similar reflector material arranged as a second chopping wheel disc can replace a second, fixed chopping wheel disc and a separate retroreflector so long as either the patterned reflective array or the chopping wheel disc rotates with respect to the other.
It will be appreciated from the foregoing summary that the invention provides a new and improved fluid-motion-powered method to modulate radiation signals from a reflector. The invention is suitable for various reflective devices, particularly for aerial and submersible reflective devices. Specifically, the invention is suitable to assist with in-flight refueling of unmanned or manned aircraft. The invention can also be used to create displays for use in positioning aircraft or other vehicles, or possibly to automate the entire refueling process or to automatically mate aerial vehicles. Thus, the invention has a wide range of use and flexibility that stems from the basic concept of the invention and broad application of reflective devices.
Fluid-motion-powered modulation of the amplitude of radiation signals is possible by capturing and manipulating the rotation caused by fluid flow. In any application of the present invention, the disadvantages of prior art systems, particularly the requirement for electrical power, is eliminated. The preferred embodiments of the present invention increase the potential application of modulated reflectors for remote position sensing and measurement by eliminating the requirements of wiring and electrical power.